INTRODUCTION

The city of Wuhan, located at the Chinese province of Hubei, experienced in December 2019 an epidemic outbreak that was initially characterized as atypical pneumonia^1,2. This disease originated on Huanan Seafood Wholesale Market, a humid commercial establishment in which various live and dead animals are commercialized^1,2. The pathogenic agent that caused the disease was identified as a coronavirus family of viruses, which was originally denominated as 2019-nCoV³, and finally receiving the name of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), while the disease which is produced by this virus is commonly known as Disease for 2019 Coronavirus (COVID-19), considered a viral zoonosis^4,5. COVID-19 is characterized for fever, cough, dyspnea, myalgias and fatigue⁶. Some patients also present digestive⁷, neurologic⁸and cardiovascular⁹problems. Almost 20% of the patients present severe complications. The most frequent of those are the pneumonia and distress respiratory syndrome on adults/6). 80% of the cases that present complications are adults over 60 years/6).

On December 30, 2019, Dr. Li Wenliang (1986-2020) alerted a Wuhan group of colleagues about the COVID-19 outbreak, a disease that later cost him his life¹⁰. Chinese authorities notified the epidemic outbreak just on December 31, 2019 and Huanan Market was closed on January 1, 2020^1,2. On January 11, 2020, the first death in China was produced and in mid January, massive migration to celebrate Chinese New Year (Lunar New Year)¹¹accelerated the contagion. Likewise, some cases started to be presented in Thailand, Japan and South Korea. On January 30, 2020, the World Health Organization (WHO) declared a public health emergency of international interest¹². Since that date, COVID-19 cases have been presented all around the world¹³, and the largest number of deaths has moved from China to Europe and, now, to the United States. All of the above led to the WHO declaring a pandemic on March 11, 2020¹⁴. To date (beginning of January 2021), the COVID-19 has produced about two million deaths and about ninety million infected. All of this has led the United Nations (UN) to declare that this disease constitutes the greatest test that humanity has faced since World War II¹⁵.

Since COVID-19 is a disease whose origin is related to animals, the aim of this article is to highlight the importance to be alert against these types of zoonoses, with special attention to COVID-19.

CORONAVIRUSES CHARACTERISTICS

The coronaviruses (CoVs) are obligate intracellular parasites that take the control of cellular machinery of the host in order to make viral copies and then to spread¹⁶. The CoVs are pathogenic agents that have threatened human beings during thousands of years¹⁷. They are denominated this way because of the corona (crown) form that they possess when observed at electronic microphotography¹⁸. In the thirties, the first CoVs in animals were identified, and decades later other infection causes in humans were identified¹⁷. CoVs belong to the Nidovirales order, Coronaviridae family, Coronavirinae subfamily/19). There exist four types of coronavirus, namely: Alfacoronavirus, Betacoronavirus, Gammacoronavirus y Deltacoronavirus^19,20.

From a structural perspective, CoVs possess a diameter between 60 nm and 140 nm and possess projections similar to spicules on their surface¹⁸. CoVs also possess non segmented genomes that share a similar organization¹⁷, which consists of a single stranded positive sense RNA (+ssRNA) (~ 30 knt) with 5’-cap structure and 3’-poly-A tail²⁰. About two-thirds of its genome contains two large overlapping open reading frames (ORF1a and ORF1b), which are translated into the replicase polyproteins pp1a and pp1ab¹⁷. Polyproteins are further processed in order to generate 16 non-structural proteins, which are designated nsp1 ~ 16. The remaining portion of the genome contains open reading frames for structural proteins, including spike glycoprotein (S), envelope protein (E), membrane protein (M) and nucleoprotein (N)¹⁷. Several lineage specific accessory proteins are also encoded according to the different existing lineages of CoVs¹⁷.

In the case of SARS-CoV-2, it was originally identified as a spherical virus with certain pleomorphic³.The virus particles possess very distinct spicules, of approximately 9 to 12 nm, granting to virions a solar crown appearance³, typical of the CoVs. The phylogenetic analysis determined that this pathogenic agent is part of the Betacoronavirus genre, Sarbecovirus (2B group)³. Research shows that SARS-CoV-2 RNA sequence consists of 29 870 nucleotides of longitude (about 9744 amino acids)²¹. Its genetic sequence is almost identical to that of SARS-CoV (which caused the Severe Acute Respiratory Syndrome epidemic in 2003), sharing 79.6% of the sequence identity with it²².

SARS-CoV-2 is able to infect pneumocytes type 2 and ciliated bronchial epithelial cells in human beings⁴, which show the angiotensin-converting enzyme protein (ACE2). It is reported that SARS-CoV-2 enters to these respiratory system cells thanks to a receptor-binding domain that is located in the spike glycoprotein (S)²³, a domain that is recognized and binds to the peptidase domain extracellular ACE2 mainly through polar residues²³. It is known that ACE2 is expressed at digestive epithelium level⁷and brain tissues (being specifically detected on glial and neuronal cells)⁸. This is why these tissues constitute a potential objective for SARS-CoV-2.

COVID-19 AS A ZOONOSIS

A zoonosis is an infectious disease in which causal pathogenic agents possess, in general, a wild animal reservoir²⁴, and from this animal reservoir, which is generally asymptomatic, the pathogen can be directly transmitted to human beings or domestic animals, who in turn can transmit it to human beings²⁴. It is demonstrated that the seven human CoVs have a zoonotic origin, and depending on the virus, exists different reservoirs and intermediaries¹⁷. Regarding the above, a great amount of research has previously warned of the big zoonotic potential and permanent epidemic risk that is constituted in CoVs that are present in some species of bats^25-27.

SARS-CoV-2 is considered as zoonotic^4,13,28,29. It is known that this coronavirus is 96% identical in complete genome level in comparison to a horseshoe bat coronavirus (Rhinolophus affinis), who lives in some regions of China and that has been proposed as its reservoir^22,30)(Figure 1). With respect to the intermediary host, there exists a lot of interest in identifying it³⁰. It has been found that Malayan pangolins (Manis javanica) possess a coronavirus whose genome has between 85.5% to 92.4% of sequence similarity with SARS-CoV-2, which has led to propose that it may be the intermediary³¹.

Recent investigation proposes the possibility that ferrets and cats may get infected with SARS-CoV-2 and transmit it again to human beings^32,33, continuing with a zoonotic cycle with the greatest difficulty to control the spread of COVID-19 in these animals³³. The above mentioned must be taken into account, especially with respect to common pets as cats. However, more research is required in order to determine effectively this possibility of contagion.

CONCLUSIONS AND FUTURE PROJECTIONS

Nowadays, various public and private organizations are participating in a real race in order to develop effectives vaccines against SARS-CoV-2 as well as drugs which help on COVID-19 treatment³⁸. It is important that the international biomedical community recognizes that COVID-19 is a zoonosis, and an unequivocal determination of which are the species that are its natural reservoir and its intermediary is extremely important, both to obtain future vaccine and to develop drugs to this disease. In that sense, the intermediary determination is useful in order to know in greater detail how the coronaviruses behave in the hosts and how do they transmit, since, in the future, we are not excent to produce a pandemic outbreak of zoonotic pathogenic agents with similar characteristics^26,30.

In consideration of all the above mentioned in this article it is concluded that is important for the Unique Health approach to be known, diffused and integrated for every health center and sanitary professionals, since given the seriousness of the zoonoses as COVID-19, we should not forget that human health is linked with animal health, and these two with environmental health.

Recently, certain risk groups have been immunized with vaccines; however, they will be available to the general population in several months. Before these projections, health organisms, both public and private, as well as the educational institutions should put strict emphasis on the measures that prevent future contagion^(5,13). Meanwhile, in order to prevent the accelerated propagation of coronavirus, it is important to maintain effective hygienic and preventive personal distancing measures³⁸.

Figure 1. SARS-CoV-2 coronavirus is a zoonotic agent that has been transmitted from animals to humans. Figure created with computer program BioRender.com